Normally the temperature of a refrigeration system is controlled by measuring the temperature of the air being present in or near a display case of the refrigeration system and controlling a flow of refrigerant to an evaporator belonging to that display case in such a way that the air temperature is maintained within a desired temperature range. Thus, in case the air temperature increases above the desired temperature range, e.g. due to an increase in the temperature of the ambient air or a defrost of the evaporator of the display case, this temperature increase will subsequently be compensated by an increase in the flow of refrigerant through the evaporator of the display case. Similarly, a decrease in the air temperature below the desired temperature range will be compensated by a decrease in the flow of refrigerant through the evaporator of the display case.
In order to maintain a high quality for as long as possible the products should be stored at a temperature which is within the desired temperature range. A deviation from this temperature range will result in a faster decay in the quality level of the products. How much faster the decay will be depends on a number of factors, such as the kind of product, how large the deviation is, for how long the temperature deviates, whether the temperature is above or below the preferred temperature range, and the composition and humidity of the ambient air. For example, food which needs to be maintained at a low temperature will decay if the temperature is too high for a period of time, and the higher the temperature and the longer the time period, the faster the quality of the food product will decay. Some products, e.g. most vegetables and some kinds of medicine, will be more or less destroyed if they are subject to temperatures below 0° C. Thus, the quality of such products will decay very rapidly if the temperature drops below 0° C. The quality decay will also depend on the ability of the product to maintain a substantially invariant temperature during a short period of time where the temperature of the surrounding air varies, i.e. it will depend on the thermodynamic properties of the product. Thus, a product having a high thermal capacity, such as a frozen chicken or a carton of milk, will be less affected by a change in the temperature of the surrounding air than a product having a relatively low thermal capacity, such as lettuce or sliced meat.
Due to the many factors mentioned above, controlling the temperature in a refrigeration system purely on the basis of the temperature of the air surrounding the products being refrigerated, i.e. without taking account of special properties of the specific product(s) being refrigerated, will not be sufficient to ensure that a high quality of the product(s) is maintained over the longest possible period of time.
CliniSense Corporation has developed an electronic time-temperature indicator and logger for logging and indicating the quality of a product. The apparatus is positioned next to the product in question and measures the temperature of the surrounding air. Based on the measured temperature and the development of this temperature over time as well as knowledge about various properties of the product, the apparatus performs a stability calculation resulting in a value which is indicative of the present quality of the product. The result of the calculation is displayed on the apparatus. Thus, when a user needs to use the product, he or she can gain information regarding the quality of the product, e.g. in the form of a symbol indicating that the product is fresh or expired, or how much shelf life there is remaining. However, it is not possible to use this information actively so as to prevent or reduce a decrease in quality of the product. A presentation of CliniSense's apparatus can be found on http://www.clinisense.com/eTTI.htm.
It is desirable to be able to control a refrigeration system in such a way that a decrease in quality of products being refrigerated is prevented or at least reduced considerably as compared to known refrigeration systems.